[PATCH] ARM: mm: only adjust sections of valid mm structures

[Doug Berger]

A timing hazard exists when an early fork/exec thread begins
exiting and sets its mm pointer to NULL while a separate core
tries to update the section information.

This commit ensures that the mm pointer is not NULL before
setting its section parameters. The arguments provided by
commit 11ce4b33aedc ("ARM: 8672/1: mm: remove tasklist locking
from update_sections_early()") are equally valid for not
requiring grabbing the task_lock around this check.

Fixes: 08925c2f124f ("ARM: 8464/1: Update all mm structures with section 
adjustments")
Signed-off-by: Doug Berger <open...@gmail.com>
---
 arch/arm/mm/init.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index be0b42937888..bdc70dff477b 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -616,7 +616,8 @@ static void update_sections_early(struct section_perm 
perms[], int n)
                if (t->flags & PF_KTHREAD)
                        continue;
                for_each_thread(t, s)
-                       set_section_perms(perms, n, true, s->mm);
+                       if (s->mm)
+                               set_section_perms(perms, n, true, s->mm);
        }
        set_section_perms(perms, n, true, current->active_mm);
        set_section_perms(perms, n, true, &init_mm);
-- 
2.7.4